1. Field of the Invention
This invention relates generally to an apparatus for generating electric power downhole within an earth borehole. More specifically, this invention relates to a downhole apparatus for generating variable electric output by varying the alignment of permanent magnets rotating within an armature having electrically conductive windings.
2. Description of the Related Art
In the field of petroleum well drilling and logging, recent advancements in drilling and logging technology have produced tools that require increasingly higher levels of electric energy downhole. Moreover, for many modern drilling and logging systems, the electric energy requirements vary over a wide dynamic range of system operating conditions. Thus, during certain operating conditions, such systems require reduced electric energy, and the dissipation of any excess electric energy must not result in destructive effects within the generator and/or the associated regulating electronics. Additionally, for a generator driven by a rotating shaft from a conventional mud-powered turbine, the rotational speed of the input shaft ("input RPM") often varies over a wide range, which, for a conventional permanent magnet generator, presents significant difficulties with respect to the dissipation of excess energy because such dissipation must occur in a downhole environment that typically involves elevated ambient temperatures.
Before the present invention, existing downhole electric generators were not directed to meeting these variable energy requirements. For example, U.S. Pat. No. 3,970,877, issued to Russell et al. on Jul. 20, 1976, discloses a method for generating downhole electric energy using a means responsive to turbulence in the drilling mud flow to convert vibratory motion into an electrical output. However, the method of the '877 patent is directed to low-power generation rather than high-power generation. Another drawback of the method of the '877 patent is that it requires electronic devices to rectify and smooth the electrical output, which is initially in the form of relatively high-voltage pulses.
U.S. Pat. No. 4,396,071, issued to Stephens on Aug. 2, 1983, discloses an apparatus for regulating the electrical output produced by a conventional mud-powered turbine by means of a by-pass valve to control the amount of mud flow passing through the turbine. Although the '071 apparatus is directed to providing relatively constant electrical output to meet the electrical demands of a downhole measurement while drilling system, the '071 apparatus attempts to accomplish that goal indirectly by controlling the input RPM to the electrical generator rather than directly controlling the electrical output of the generator regardless of the input RPM, which would be more desirable. Additionally, the by-pass valve of the '071 apparatus would suffer from the destructive effects of erosion that are frequently encountered in the use of typical drilling fluids (muds). Similarly, U.S. Pat. No. 4,491,738, issued to Kamp on Jan. 1, 1985, discloses a machine for generating electrical energy by controlling drilling fluid dynamics, preferably in response to fluid pressure changes created by a conventional mud-pulse telemetry system, to move a reciprocating anchor comprising a plurality of magnets inside a stator, and U.S. Pat. No. 4,515,225, issued to Dailey on May 7, 1985, discloses an apparatus in which a fluid separate from the drilling mud is used to activate an electrical generator. Again, however, the '738 and '225 machines are not directed to meeting the aforementioned variable electrical requirements.
It would, therefore, be a significant advancement in the art to provide an improved downhole apparatus for generating variable electrical output over a wide range of input RPM and downhole system electrical requirements.